drm/omap: dsi: use pixel-format and mode from attach

In order to reduce the amount of custom functionality, this moves
handling of pixel format and DSI mode from set_config() to dsi
attach.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-22-tomi.valkeinen@ti.com